One explanation for Vice President Joe Biden’s gaffe-prone reputation? He just isn’t very bright, said former New York City Mayor Rudy Giuliani.

The former GOP presidential hopeful had harsh words for Biden in the wake of a statement the vice president made yesterday, when Biden told a Virginia audience with a large African-American presence that Republicans and Wall Street will “put y’all back in chains.”

“I think if it came from somebody serious, maybe we’d get all excited about it,” Giuliani said on CNBC’s The Kudlow Report, dismissing Biden. “But I think the vice president of the United States has become a laugh line on late night television. I mean, I’ve never seen a vice president that has made as many mistakes, said as many stupid things. … There’s a real fear if, God forbid, he ever had to be entrusted with the presidency, whether he really has the mental capacity to handle it.

“I mean, this guy just isn’t bright. He’s never been bright. He isn’t bright. People think, ‘Well, he just talks a little too much.’ Actually, he’s just not very smart.”

Biden, for his part, clarified yesterday that he was using the Republican ticket’s own words. “Here’s what Congressman Ryan said,” according to a statement from Biden. “He said, ‘We believe a renewed commitment to limited government will unshackle our economy.’… The last time these guys unshackled the economy, to use their term, they put the middle class in shackles. That’s how we got where we are.”

Giuliani, who initially supported Florida Sen. Marco Rubio for the vice presidential nod, issued a staunch defense of the Romney-Ryan ticket, inadvertently offering the most coveted comparison a conservative can receive.

“If we get the debate as a choice between Obama’s vision and the Romney-Reagan — I was going to say Reagan, the Romney-Ryan vision, because he reminds me a bit of Ronald Reagan — I think we’ll win this election. It is a gamble, but I think it’s a gamble that’s going to work out.”